The automatic postural adjustments maintaining human balance will be studied in freely standing subjects. The overall aim of the project is to test a concept for how the nervous system coordinates muscle contractions controlling the leg, trunk, and head components of the postural movements. Specifically, we postulate that muscle contractions are organized by a limited repertoire of discrete muscle synergies. Postural movements are then adapted to a variety of different conditions by different temporal and spatial combinations of the muscle synergies. Subjects will stand on a movable platform system. Postural adjustments to a variety of different movement perturbations will be described by analyzing the EMG activity of selected leg, lower trunk, and neck muscles and by recording the motions of the ankle, knee, hip, and neck angles. Experiments are designed to test the hypothesis that the appropriate combination of muscle synergies is selected based upon information of body segment positions, while the amplitude and timing of the movement is controlled by velocity information. Studies will include normal adult subjects and select groups of patients with peripheral vestibular deficits, Parkinson's disease, and cerebellar deficits. Patient studies will describe movement abnormalities in terms of ability to organize the individual muscle synergies, to select the appropriate combination of synergies, and to generate the appropriate amplitudes of response.